Fuel-cascaded fuel cell stacks with decoupled power outputs

Abstract

Fuel exhaust ( 109 ) of a primary fuel cell stack ( 11 ) flows into an auxiliary fuel cell stack ( 12 ) which powers a DC storage ( 82 ) feeding a bi-directional DC/AC converter ( 86 ) that is switchable ( 89 ) to auxiliary equipment ( 90, 91 ) (such as pumps) to a main power bus ( 54 ) feeding a main load ( 55 ). Fresh fuel ( 97 ) is provided ( 98, 105 ) to the primary stack for 90% fuel utilization, with over 99% overall power plant fuel utilization. The auxiliary equipment ( 90, 91 ) may be powered by the bus ( 54 ).

1 . A method characterized by:
 coupling ( 28 ,  108 - 110 ) fuel exhaust of a primary fuel cell stack ( 11 ) to a fuel inlet ( 37 ) of an auxiliary fuel cell stack ( 12 ) not connected in series voltage relationship with said primary fuel cell stack; and   operating ( 65 ) said primary fuel cell stack and said auxiliary fuel cell stack in a manner that avoids fuel starvation in said primary fuel cell stack and that results in substantially all of the fuel provided to said primary fuel cell stack being consumed by said fuel cell stacks without regard to exposing said auxiliary fuel cell stack to fuel starvation.   
   
   
       2 . A method according to  claim 1  characterized in that said step of operating comprises providing fuel ( 97 ,  98 ,  105 ,  108 ) to said primary fuel cell stack to achieve fuel utilization in said primary fuel cell stack of about 90%. 
   
   
       3 . A method according to  claim 1  characterized in that said step of operating results in about 99% or more of the fuel provided to said primary fuel cell stack being consumed by said fuel cell stacks. 
   
   
       4 . A method according to  claim 1  further characterized by:
 storing electric power ( 35 ,  36 ,  73 ,  79 ,  80 ) generated by said auxiliary fuel cell stack ( 12 ) in an energy storage device ( 82 ).   
   
   
       5 . A method according to  claim 4  further characterized by:
 providing ( 86 ,  89 - 91 ) electric power from said energy storage device ( 82 ) to operate at least one pump ( 91 ) selected from (a) a reactant pump configured to supply a reactant gas to at least one stack selected from said primary fuel cell stack and said auxiliary fuel cell stack, or (b) a coolant pump configured to supply coolant to at least one stack selected from said primary fuel cell stack and said auxiliary fuel cell stack.   
   
   
       6 . A method according to  claim 4  further characterized by:
 selectively ( 89 ) providing electric power from either (a) a power bus ( 54 ) that provides power from said primary fuel cell stack ( 11 ) to a main load ( 55 ) or (b) said energy storage device ( 82 ), to operate at least one pump selected from (a) a reactant pump configured to supply a reactant gas to at least one stack selected from said primary fuel cell stack and said auxiliary fuel cell stack, and (b) a coolant pump configured to supply coolant to at least one stack selected from said primary fuel cell stack and said auxiliary fuel cell stack.   
   
   
       7 . A method according to  claim 4  further characterized by:
 selectively conducting ( 77 ,  86 ,  89 ) power either (a) from said energy storage device ( 82 ) to a power bus ( 54 ) that provides power from said primary fuel cell stack ( 11 ) to a main load ( 55 ) of said power plant, or (b) to said energy storage device from said power bus.   
   
   
       8 . A method according to  claim 4  further characterized by:
 said steps of operating and storing comprise storing electric power ( 35 ,  36 ,  73 ,  79 ,  80 ) generated by said auxiliary fuel cell stack ( 12 ) in said energy storage device ( 82 ) and ceasing to store electric power generated by said auxiliary fuel cell stack in immediate response to the amount of fuel being provided from said primary fuel cell stack to said secondary fuel cell stack being sufficiently low so as to render fuel starvation in said secondary stack likely to occur.   
   
   
       9 . A method according to  claim 1  further characterized by:
 providing ( 77 ,  97 ,  98 ,  110 ,  113 ) fresh fuel to said auxiliary fuel cell stack ( 12 ) prior to start-up of said primary fuel cell stack ( 11 ).   
   
   
       10 . A method according to  claim 1  further characterized by:
 providing ( 77 ,  97 ,  98 ,  110 ,  113 ) fresh fuel to said auxiliary fuel cell stack ( 12 ) after start up of said primary fuel cell stack ( 11 ) to avoid fuel starvation in said auxiliary fuel cell stack.   
   
   
       11 . A method according to  claim 1  further characterized by:
 providing ( 77 ,  97 ,  98 ,  110 ,  113 ) fresh fuel to said auxiliary fuel cell stack ( 12 ) during normal operation of said primary fuel cell stack ( 11 ) to avoid fuel starvation in said auxiliary fuel cell stack.   
   
   
       12 . A method according to  claim 1  further characterized in that said step of operating includes providing more than stoichiometric amount of air to each of said stacks ( 11 ,  12 ) and ceasing to provide air to said auxiliary stack in immediate response to the amount of fuel being provided from said primary fuel cell stack to said secondary fuel cell stack being sufficiently low so as to render fuel starvation in said secondary stack likely to occur. 
   
   
       13 . A method according to  claim 1  further characterized by:
 said step of operating said fuel cell stacks includes electrically loading said primary fuel cell stack independently of electrically loading said auxiliary fuel cell stack.   
   
   
       14 . A method according to  claim 1  characterized in that said step of operating comprises operating said primary fuel cell stack ( 11 ) and said secondary fuel cell stack ( 12 ) in a manner that sacrifices said secondary fuel cell stack to the effects of fuel starvation in order to protect said primary fuel cell stack from degradation. 
   
   
       15 . A fuel cell power plant ( 10 ) comprising:
 a primary fuel cell stack ( 11 );   characterized by:   an auxiliary fuel cell stack ( 12 ) not connected in serial voltage relationship with said primary fuel cell stack;   means ( 28 ,  108 - 110 ) configured to couple fuel exhaust of said primary fuel cell stack to a fuel inlet of said secondary fuel cell stack; and   means ( 65 ) configured to operate said primary fuel cell stack and said auxiliary fuel cell stack in a manner that avoids fuel starvation in said primary fuel cell stack and that results in substantially all of the fuel provided to said primary fuel cell stack being consumed by said fuel cell stacks without regard to exposing said auxiliary fuel cell stack to fuel starvation.   
   
   
       16 . A power plant ( 10 ) according to  claim 15  further characterized by:
 an electric energy storage device ( 82 ); and   means ( 35 ,  36 ,  73 ,  79 ,  80 ) configured to store electric power generated by said auxiliary fuel cell stack ( 12 ) in said electric energy storage device.   
   
   
       17 . A power plant ( 10 ) according to  claim 16  further characterized by:
 means ( 86 ,  89 - 91 ) configured to provide electric power from said energy storage device ( 82 ) to operate at least one pump ( 91 ) selected from (a) a reactant pump configured to supply a reactant gas to at least one stack selected from said primary fuel cell stack ( 11 ) and said auxiliary fuel cell stack ( 12 ), or (b) a coolant pump configured to supply coolant to a stack selected from said primary fuel cell stack and said auxiliary fuel cell stack.   
   
   
       18 . A power plant ( 10 ) according to  claim 16  further characterized by:
 means ( 89 ) configured to selectively provide electric power from either (a) a power bus ( 54 ) that provides power from said primary fuel cell stack ( 11 ) to a main load ( 55 ) of said power plant or (b) said energy storage device ( 82 ), to operate at least one pump ( 91 ) selected from (a) a reactant pump configured to supply a reactant gas to a stack selected from said primary fuel cell stack and said auxiliary fuel cell stack, or (b) a coolant pump configured to supply coolant to a stack selected from said primary fuel cell stack and said auxiliary fuel cell stack.   
   
   
       19 . A power plant ( 10 ) according to  claim 16  further characterized by:
 means ( 86 ,  89 ) configured to selectively conduct power either (a) from said electric storage device ( 82 ) to a power bus ( 54 ) that provides power from said primary fuel cell stack ( 11 ) to a main load ( 55 ) of said power plant, or (b) from said power bus to said electric storage device.   
   
   
       20 . A power plant ( 10 ) according to  claim 15  further characterized by:
 means ( 77 ,  97 ,  98 ,  110 ,  113 ) configured to provide fresh fuel to said auxiliary fuel cell stack ( 12 ) prior to start-up of said primary fuel cell stack ( 11 ).   
   
   
       21 . A power plant ( 10 ) according to  claim 15  further characterized by:
 means ( 77 ,  97 ,  98 ,  110 ,  113 ) configured to provide fresh fuel to said auxiliary stack after start up of said primary fuel cell stack ( 12 ) to avoid fuel starvation in said auxiliary fuel cell stack ( 11 ).   
   
   
       22 . A power plant ( 10 ) according to  claim 15  further characterized by:
 means ( 77 ,  97 ,  98 ,  110 ,  113 ) configured to provide fresh fuel to said auxiliary stack during normal operation of said primary fuel cell stack ( 12 ) to avoid fuel starvation in said auxiliary fuel cell stack ( 11 ).   
   
   
       23 . A power plant ( 10 ) according to  claim 15  further characterized by:
 said auxiliary fuel cell stack ( 12 ) having less power output capability than said primary fuel cell stack ( 11 ).   
   
   
       24 . A power plant ( 10 ) according to  claim 15  further characterized by:
 said auxiliary fuel cell stack ( 12 ) having a different number of fuel cells ( 11   a ) than the number of fuel cells ( 11   a ) in said primary fuel cell stack ( 11 ).   
   
   
       25 . A power plant ( 10 ) according to  claim 24  further characterized by:
 said auxiliary fuel cell stack ( 12 ) having a fewer number of fuel cells ( 11   a ) than the number of fuel cells ( 11   a ) in said primary fuel cell stack ( 11 ).   
   
   
       26 . A power plant ( 10 ) according to  claim 15  further characterized by:
 the fuel cells ( 11   a ) in said primary fuel cell stack ( 11 ) have active areas different than the active areas of the fuel cells ( 12   a ) in said secondary fuel cell stack ( 12 ).   
   
   
       27 . A power plant ( 10 ) according to  claim 26  further characterized by:
 the fuel cells ( 11   a ) in said primary fuel cell stack ( 11 ) have active areas larger than the active areas of the fuel cells ( 12   a ) in said secondary fuel cell stack ( 12 ).   
   
   
       28 . A power plant ( 10 ) according to  claim 15  further characterized by:
 said fuel cell stacks ( 11 ,  12 ) being configured in a single contiguous structure.   
   
   
       29 . A power plant ( 10 ) according to  claim 28  further characterized by:
 said fuel cell stacks ( 11 ,  12 ) being configured with either (a) a cathode end of the primary stack ( 11 ) contiguous with a cathode end of the secondary stack, or (b) an anode end of the primary stack contiguous with the an anode end of the secondary stack.   
   
   
       30 . A power plant ( 10 ) according to  claim 15  characterized in that said means ( 65 ) configured to operate said primary fuel cell stack ( 11 ) and said secondary fuel cell stack ( 12 ) operates said fuel cell stacks in a manner that sacrifices said secondary fuel cell stack to the effects of fuel starvation in order to protect said primary fuel cell stack from degradation.